This invention relates to vapor-deposited films, and to controlling stress in such films. Vapor-deposited films are used, for example, in certain micro-electro-mechanical systems (“MEMS”) devices. MEMS devices typically are fabricated using integrated circuit fabrication technology, and include movable structures such as tilting mirrors. A typical MEMS tilting mirror may include a conductive member and be resiliently mounted to a base. The base and the movable structure of a MEMS device are typically made of silicon. The base includes one or more fixed conductive electrodes that are supported, and are insulated from each other and from the movable conductive member, by dielectric material. Applying a potential difference between the movable conductive member and one or more of the fixed electrodes produces an attractive electrostatic force urging the movable structure toward the fixed electrodes. The resilient mounting (e.g., one or more springs) of the movable structure provides a restoring force.
In certain applications, a thin film may be applied to a portion of a MEMS device, such as by vapor deposition. For instance, in some optical MEMS devices such as optical cross-connects, the movable structure includes a micromirror having a reflecting surface in order to redirect an incident light beam in a controllable direction. The reflectivity of the material from which the movable structure is formed may be insufficient to provide an adequate reflecting surface in certain applications. In such applications, the material from which the movable structure is formed may be used as a structural element to provide a substrate for a reflecting surface, and another material may be disposed on the movable structure in order to provide the reflecting surface. For instance, in certain applications, the reflectivity of silicon may be inadequate, but the reflectivity of gold may be adequate; a thin film of gold deposited on a silicon movable structure substrate would provide it with an adequate reflecting surface.
However, gold adheres poorly to silicon. In order to adhere a gold layer to a silicon movable structure, a layer of material that adheres well to silicon and to gold may be disposed on the silicon movable structure as an “adhesion layer,” and the gold layer may be disposed on the adhesion layer. Titanium is commonly used as an adhesion layer in composite thin film stacks, and is suitable for use as an adhesion layer between silicon and gold.
One of the important characteristics of a reflective surface for an optical cross-connect, as well as many other devices, is flatness; the reflective surface should be flat when the device is manufactured, and should remain flat for the lifetime of the device. However, mechanical stresses in metallic thin films applied to a substrate material of a MEMS device movable structure can cause the substrate to deform by bending, and the thin film, which conforms to the shape of the movable structure substrate, likewise bends. The change in the shape of the reflective surface due to film stress usually results in reduced optical performance. This is a particularly serious concern if the stress in the metallic films changes over time, for instance, due to creeping or if thermal expansion mismatch leads to additional stress buildup during device processing steps subsequent to film deposition.
The stress of a titanium layer can be reduced by decreasing its thickness. However, if a titanium layer is too thin, it becomes ineffective as an adhesion layer. When a titanium layer included in a thin film stack is thick enough to be suitable as an adhesion layer, its stress usually dominates the total stress of the stack. A number of ways have been suggested to reduce the effect of stress in a titanium film in a MEMS device.
One approach is to metallize both sides of a micromirror. If the metal films deposited on both sides of the micromirror were identical, their stresses would cancel each other and the micromirror's curvature would remain that same as it was prior to metallization. In practice, however, balancing the stress is problematic. Not only must the initial stress be exactly balanced, but the stress of the two metal films must also relax at the same rate so that they remain balanced over time. Any deviations from this ideal behavior may lead to curvature that deteriorates with time.
Optical MEMS devices frequently are made from two chips: a mirror chip having an array of movable micromirrors and an electrode chip having a correspondingly spaced array of driving electrode structures. In the device assembly process the two chips are brought together in registration and adhered to each other. Once the two chips are adhered to each other, the side of the micromirrors facing the electrode chip is inaccessible for metallizing, and so a device in which both sides of a mirror are metallized requires that the metallizing occur prior to adhering the mirror chip to the electrode chip. Therefore, the metal films deposited on both sides of such mirrors will be subjected to the assembly and packaging processes, which may involve relatively high temperatures. Because of relaxation at high temperatures and thermal expansion mismatch, the metal films pick up a considerable amount of tensile stress upon cooling to room temperature.
Another approach to reducing stress effects due to films deposited on micromirror substrates is robust mechanical design. For instance, to reduce the bending caused by a given amount of film stress, the substrate thickness could be increased. While a thicker substrate improves stress-induced curvature, it affects the mechanical properties of the micromirror, such as by lowering its resonant frequency, and it complicates the design of the springs that support the micromirror and allow it to tilt.
Balancing of stress by additional thin films to make the net bending moment zero is disclosed in “Curvature Compensation in Micromirrors With High-Reflectivity Optical Coatings,” K. Cao, W. Liu, and J. Talghader, Journal of Microelectromechanical Systems, Vol. 10, No. 3, September 2001, 409-417. It has also been suggested to add thermal expansion-induced stress that compensates intrinsic thin film stress. These approaches add complexity to the fabrication process, and do not change the intrinsic thin film stress of the functional metal. In addition, the application of these techniques may be limited because of restrictions in mechanical layout, thermal budget, and stability as a function of time.